Chinese Utility Model No. 201474570U discloses a conventional built-in blind assembly including front and rear panels, a blind unit disposed between the front and rear panels, opposite top and bottom frames disposed respectively above and below the blind unit and between the front and rear panels, and a U-shaped top rail seat removably coupled to the top frame and cooperating with the top frame to define an accommodating cavity for receiving a cord-driving unit therein. However, since an open side of the accommodating cavity faces upwardly toward the top frame, and since the top rail seat is coupled to the top frame, it is not possible for a user to access the cord-driving unit in the accommodating space without decoupling the top frame and the top rail seat first. Moreover, when assembling the conventional blind assembly, vision of the user may be blocked by the top rail seat, thereby resulting in a relatively low assembling efficiency.